Traditional masonry wood burning fireplaces comprise a large masonry firebox and a masonry chimney which extends upwardly above the roof of a house to vent the combustion gasses formed when the wood burning fireplace is used. These fireplaces have been and continue to be very popular. Unfortunately, they suffer from a number of drawbacks. For example, such fireplaces are fixed in place and require the room to be built and decorated around the fireplace. Because their heating efficiency is also very low, they do not make very good room heaters. Moreover, they need to be cleaned continuously due to ash accumulation.
Factory built gas fireplaces on the other hand overcome many of the drawbacks suffered by the traditional masonry fireplaces. Depending on the design, gas fireplaces can be highly efficient and hence good room heaters. They may be free standing units which can be located anywhere in a room, zero clearance units designed to abut a wall or fireplace inserts which fit into existing masonry fireboxes. As used herein "factory built gas fireplaces" is meant to include free-standing gas fireplaces, zero clearance gas fireplaces and gas fireplace inserts using natural, propane or butane gas.
Most gas fireplaces require venting, generally from the top or rear of the fireplace. For example, rear direct vent fireplaces comprise an intake air vent and an exhaust vent which extend rearwardly through an exterior wall of the room in which the unit is located.
Gas fireplaces typically comprise room air plenums below, adjacent the sides and/or rear and above the firebox. A convection current is created whereby room air is pulled into the bottom plenum, passes upwardly in the side and/or rear plenums into the top plenum and thereafter exits the fireplace from the top plenum. As the room air circulates around the firebox, heat is transferred from the firebox to the circulating room air so that the temperature of the air exiting the top plenum, is substantially increased.
In a direct vent gas fireplace, combustion air from outside the house or building is delivered to the combustion chamber typically through an intake vent and an intake plenum which extends behind the firebox and, in some cases, below the firebox. Combustion air from the intake plenum passes into the combustion chamber through one or more openings in the bottom wall or at the bottom of the rear wall or side walls of the firebox.
In vented gas fireplaces, exhaust gas is typically passed from the combustion chamber directly into an exhaust vent. To maximize heating efficiency, some gas fireplaces employ one or more baffles within the combustion chamber to increase the residence time of the hot gasses within the combustion chamber thereby maximizing heat transfer through the glass front and through the firebox walls to room air circulating around the firebox.
Vent free units, in which exhaust gasses are vented directly into the room are also known. In such fireplaces, the combustion gasses are tightly controlled to prevent formation of excessive amounts of carbon monoxide and undesirable hydrocarbons. Such tight controls require a clean burning flame. This makes it difficult to simulate the aesthetically pleasing, but relatively dirty yellow flames of wood burning fireplaces in a ventless fireplace.